The largest application area of polyimide (PI) thin films is flexible printed circuit boards. With the rapid development of electronic products, electronic assembly technology has posed a serious challenge one after another. In order to cater to the development of electronic technology, people have made bold innovations in electronic assembly technology. Against this background, a kind of flexible circuit (flexible circuit) with fine wires and thin, flexible polymer film has emerged. It can be applied to surface mounting technology and can be bent into numerous as the shapes needed.
Flexible circuit boards (FPCs) using SMT technology can be made very thin, very delicate, insulation thickness less than 25 microns, this flexible circuit can be arbitrarily bent and can be curled into a cylinder, to make full use of the three-dimensional volume. It breaks the traditional thinking of the inherent use area, thus forming the ability to make full use of the volume shape. This can significantly enhance the effective use density and form a high-density assembly form on the conductor length used in each unit area. In recent years, the use of flexible circuits has been extended to radio communications, computers and automotive electronic equipment and other fields. In the past, flexible circuits were used exclusively as substitutes for rigid wires and cables. They have been mature enough to be used as substitutes for rigid circuits and printed circuit boards in applications requiring thin or three-dimensional circuits. In order to meet the application requirements of rigid-flexible combination, rigid-flexible technology is combined with flexible circuit (rigid-flexible printed circuit board).
The main materials used in general flexible circuits (three-layer FPC) are insulating films (insulating layers), adhesives and conductors (conducting layers). The insulating film forms the basis of the circuit and forms the insulating layer of the flexible circuit substrate. The adhesive bonds the copper foil to the insulating film, and many layers are bonded together in the multilayer structure design. An outer protective layer (covering film) is used to isolate electricity from sand, dust and moisture, while reducing the stress during flexure. The conductive layer is provided by copper foil. In some flexible circuits, reinforcement plates (consisting of aluminum, stainless steel, composite materials, etc.) are used as reinforcement ribs to ensure geometric stability. At the same time, it can also provide mechanical support force when components and connectors are inserted, and eliminate stress.
At present, flexible substrate materials used in flexible printed circuit boards are generally flexible copper clad laminate (FCCL), FCCL is divided into two types: adhesive type (i.e. three-layer, 3L-FCCL) and non-adhesive type (i.e. two-layer, 2L-FCCL). Although the types of FCCL are different, most of the insulation films used in FCCL are polyimide (PI) films. At the same time, in the process of manufacturing flexible printed circuit boards, rigid-flexible printed circuit boards, in addition to the use of FCCL and PI film has a great demand, but also in the above-mentioned cover film, reinforcement board PI film (or PI film production products). FCCL manufacturers generally produce all three kinds of flexible materials at the same time in order to supply them to FPC manufacturers.